For example, a solid polymer electrolyte fuel cell employs a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is a polymer ion exchange membrane. In the fuel cell, the solid polymer electrolyte membrane is interposed between an anode and a cathode each comprising an electrode catalyst layer and porous carbon to form a membrane electrode assembly (electrolyte electrode assembly). The membrane electrode assembly is sandwiched between separators (bipolar plates) to form a unit cell. In use, normally, a predetermined number of unit cells are stacked together to form a fuel cell stack.
In general, the fuel cell adopts so called internal manifold structure where supply passages and discharge passages extend through the separators in the stacking direction. The fuel gas, the oxygen-containing gas, and the coolant are supplied from the respective supply passages to a fuel gas flow field, an oxygen-containing gas flow field, and a coolant flow field, and then, discharged to the respective discharge passages.
Therefore, in the fuel cell, it is necessary to prevent leakage of the fuel gas, the oxygen-containing gas, and the coolant individually. In this regard, for example, a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2002-270202 is known. In the fuel cell stack, as shown in FIG. 45, fuel cells 1001 are stacked together, and each of the fuel cells 1001 is formed by sandwiching an electrode assembly 1002 between a first separator 1003 and a second separator 1004.
The electrode assembly 1002 includes an anode 1002b, a cathode 1002c, and an electrolyte membrane 1002a interposed between the anode 1002b and the cathode 1002c. The surface area of the cathode 1002c is smaller than the surface area of the anode 1002b. The first separator 1003 and the second separator 1004 are hermetically sealed by an outer seal member 1005a, and a space between the second separator 1004 and the outer end of the electrode assembly 1002 are hermetically sealed by an inner seal member 1005b. Further, a seal member 1005c is provided between the fuel cells 1001.
As shown in FIG. 46, in the fuel cell 1001 disclosed in Japanese Laid-Open Patent Publication No. 2002-270202, a fuel gas inlet 1007a, an oxygen-containing gas inlet 1008a, and a coolant inlet 1009a are formed at one end of first and second separators 1003, 1004 in a longitudinal direction, and a fuel gas outlet 1007b, an oxygen-containing gas outlet 1008b, and a coolant outlet 1009b are formed at the other end of the first and second separators 1003, 1004 in the longitudinal direction.
In the fuel cell 1001, the dimensions of the first and second separators 1003, 1004 are relatively large in comparison with the outer dimensions of the electrode assembly 1002. In the case of adopting a structure where several hundreds of the fuel cells 1001 are stacked together and used as the fuel cell stack in a vehicle application, the overall size and weight of the fuel cell stack may become large undesirably.
Further, in the fuel cell 1001, an outer seal member 1005 and an inner seal member 1006 having desired shapes are formed beforehand, and then, the outer seal member 1005 and the inner seal member 1006 are supported, e.g., by the second separator 1004. Thus, the process of producing the fuel cell 1001 and operation of assembling the fuel cell 1001 may become complicated undesirably.
Further, in a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2002-025587, as shown in FIG. 47, a fuel unit cell 1101 is sandwiched between a first separator 1102 and a second separator 1103. The fuel unit cell 1101 is formed by sandwiching a solid polymer electrolyte membrane 1104 between a cathode 1105 and an anode 1106. The cathode 1105 and the anode 1106 include gas diffusion layers 1105a, 1106a. 
The solid polymer electrolyte membrane 1104 protrudes out from inner circumferences of the cathode 1105 and the anode 1106. The surface area of the cathode 1105 is smaller than the surface area of the anode 1106.
A first seal 1107a and a second seal 1107b are attached between the first separator 1102 and the second separator 1103. The first seal 1107a tightly contacts the solid polymer electrolyte membrane 1104, and the first seal 1107a is provided around the cathode 1105. The second seal 1107b is provided around the anode 1106, and around the first seal 1107a. Thus, leakage of the oxygen-containing gas is prevented by the first seal 1107a, and leakage of the fuel gas is prevented by the second seal 1107b. In the structure, the first seal 1107a and the second seal 1107b are provided at positions deviated laterally with respect to the stacking direction of the fuel cell. Therefore, reduction in the overall thickness of the fuel cell in the stacking direction is achieved.
In the case of the fuel unit cell 1101, at the time of stacking a plurality of the fuel unit cells 1101, a coolant flow field is formed between each fuel unit cell 1101 along the electrode surface for cooling the fuel unit cell 1101. Thus, a seal member for preventing leakage of the coolant needs to be provided between each fuel unit cell 1101. In the presence of the seal member, the fuel unit cells 1101 tend to be spaced from each other, and reduction in the overall size of the fuel cell stack may not be achieved.
In a process control apparatus disclosed in Japanese Laid-Open Patent Publication No. 06-218275, as shown in FIG. 48, stack plates each formed by overlapping two plates in parallel with each other, and units 1202 are stacked alternately. The unit 1202 is formed by sandwiching an MEA 1202a between an anode 1202b and a cathode 1202c, and sandwiching these components between a pair of contact plates 1202d. 
A first chamber 1203a is formed between the plate 1201a and the unit 1202, a second chamber 1203b is formed between the plate 1201b and the unit 1202, and a third chamber 1203c is formed between the plates 1201a, 1201b. A passage 1205 extends through ends of the plates 1201a, 1201b through packings 1204.
The passage 1205 is connected to, e.g., the second chamber 1203b through a flow field 1206 between the plates 1201a, 1201b. Though not shown, two other passages extend in the stacking direction, and the other two passages are connected to the first chamber 1203a and the third chamber 1203c, respectively through flow fields (not shown) between the plates 1201a, 1201b. 
However, in the process control apparatus, the flow field 1206 for connecting the passage 1205 extending in the stacking direction to the second chamber 1203b needs to have the sufficient flow field height in the stacking direction, and the sufficient seal height by the packing 1204, in order for sufficient fluid to flow therethrough. Thus, the space between the units 1202 becomes considerably large, and reduction in the overall size of the fuel cell cannot be achieved.
In particular, a fuel cell stack mounted in a vehicle is formed by stacking a large number of, e.g., several hundreds of fuel cells. Therefore, space between the fuel cells becomes large, and reduction in the overall size of the fuel cell cannot be achieved.